This invention relates to a fuel injection system for an engine.
A multi-point type fuel injecting system is extensively employed for an automobile engine to reduce hazardous components from the exhaust gas of the engine and to improve the output of the latter. On the other hand, in order to reduce the quantity of hazardous exhaust gas essentially containing hydrocarbon (HC), a fuel injection valve (hereinafter referred to as "an injector", when applicable) which jets fuel and atomizes it by mixing air with it, has been put in practical use; that is, a so-called "air assist injector" has been employed.
A conventional engine fuel injecting system is as shown in FIG. 3. In FIG. 3, reference numeral 1 designates an engine; 2 a suction pipe; 3, a throttle valve for controlling the quantity of air supplied into the engine 1 through the suction pipe 2; 4, an injector for jetting fuel into the suction pipe 2; 5, a bypass passageway which receives air from upstream of the throttle valve 3 and supplies it to the injector 4; 6, atomizing means for mixing air with fuel jetted from the injector 4 to accelerate atomization of the fuel; and 7, a control unit which calculates a quantity of fuel supplied to the engine and drives the injector in a pulse mode.
In order to calculate an amount of injection of fuel; that is, an injector drive pulse width, it is necessary to use a suction air sensor for detecting a quantity of suction air, an engine speed sensor, a water temperature sensor, etc. (those sensors being not shown in FIG. 3, because use of them is well known in the art). In addition, description of a quantity-of-fuel calculating process or method will not be made here.
The operation of the fuel injection system thus organized will be described. Particularly, a technique will be described in which fuel jetted from the injector 4 concerning this invention (described later) is atomized by mixing it with air. A purpose of atomization of the fuel is to reduce the quantities of hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas of the engine. It has been found through experiments that, as droplets of fuel jetted from the injector are reduced in particle size, the combustion of the fuel is improved. That is, as shown in FIG. 4, the quantity of HC or CO gas discharged is substantially proportional to the particle size of the droplets of fuel jetted from the injector.
There are available a variety of fuel atomizing methods, for instance, a fuel atomizing method of air assistance type, a fuel atomizing method of ultrasonic type, and a fuel atomizing method based on a mechanical structure. The fuel atomizing method of air assistance type concerns a fuel injection system of this invention (described later). In the conventional fuel atomizing method of air assistance type, fuel is atomized as follows: As shown in FIG. 3, air is led to the vicinity of the jetting nozzle of the injector 4 from upstream of the throttle valve 3, and the atomizing means 6 operates on the difference in pressure between two portions of the suction pipe 2 which are located upstream and downstream of the throttle valve 3, respectively, to mix the air with fuel jetted from the injector, thereby to atomize the fuel.
In the method, the quantity of assisting air relates to the particle size of droplets of fuel as shown in FIG. 5. That is, the particle size is decreased substantially in reverse proportion to the quantity of assisting air; while the number of idling revolutions per minute (rpm) is increased in proportion to the quantity of assisting air.
As for the atomization of fuel, the quantity of assisting air Q.sub.a should be Q.sub.a2. However, with Q.sub.2, the number of idling revolutions per minute is high, 800 to 100 rpm; that is, fuel is not used economically, and noises may be formed. Hence, it is not desirable to set the quantity of assisting air to Q.sub.a2. Therefore, unavoidably the sectional area of the bypass passageway 5 is so determined that the quantity of assisting air be Q.sub.a2. However, if the sectional area of the by pass passageway 5 is determined so, then the fuel will not sufficiently atomized; that is, it will be impossible to sufficiently reduce the quantity of hazardous gases in the exhaust gas of the engine.
In the conventional fuel injection system designed as described above, atomization of fuel is accelerated as the quantify of assisting air is increased; however, increasing the quantity of assisting air suffers from a problem that the number of idling revolutions per minute is increased, so that the fuel is not economically used, and noises are increased. Hence, in practice, the sectional area of the bypass passageway is so determined that the quantity of assisting air allows the number of idling revolutions per minute to be of the order of 600 to 700 rpm. In this case, atomization of the fuel is insufficient, and reduction of the quantity of HC and CO gases in the exhaust gas cannot be made to satisfaction.
On the other hand, in order to maintain the flow rate of air in the bypass passageway 5 higher than a certain value, it is necessary to reduce the leakage of air as much as possible when the throttle valve 3 is closed. However, if the leakage of air is excessively reduced, then a trouble may occur that the throttle valve 3 is caught in the throttle bore.